When developing subsea oil and gas wells there are stringent demands to the control and containment of the well during all aspects of the work, be it drilling, production or later intervention. The needs for control of well pressure have lead to requirements for safe barriers in the well and/or above ground, both during production and during intervention work.
During the lifetime of the well various types of work may be carried out to enhance production or to measure conditions in the well. Well intervention may be difficult, as existing barriers have to be removed to gain entry into the well. There are in most countries strict rules regarding the size and number of barriers needed to keep control of the well during intervention. To gain access to a living well a blowout preventer, containing a number of valves, must be connected to the well before the well barriers can be opened. In addition, a number of pressure containment devices ensure control over the well during the work.
One of the methods for gaining entry into a live well is by using a lubricator. This employs a tool attached to the end of a wire or cable and inserted into the well. This equipment includes means whereby grease can be injected under pressure to seal around and lubricate the wire or cable during rising or lowering of the tool, hence the name lubricator.
Lubricators are in use both on surface and on subsea wells. U.S. Pat. No. 4,993,492 shows an example of a surface lubricator, while U.S. Pat. No. 3,638,722 and International Patent Application no. WO 0125593 shows examples of subsea lubricators. In for example WO 0125593 there is shown a subsea lubricator consisting of the afore-mentioned blowout preventer, called a Lower Intervention Package (LIP), a tool housing (or lubricator pipe), and a pressure control head which includes a grease injector assembly. When lowering a tool into the well using this equipment, the wire or cable is inserted through the pressure control head and the tool attached to the end of the wire. Then the whole assembly is lowered to the seabed and the tool guided into the tool housing while the LIP valves and Christmas tree valves are closed. Then the grease injector is closed around the wire above the tool. The LIP valves and Christmas tree master valve can now be opened so that the tool can be lowered into the well.
The tool housing must be of a length capable of holding the full length of the tool, and this can be up to 30 meters. The whole lubricator assembly may be up to 50 meters long.
To ensure a greater degree of safety, an additional blow out preventer is mounted on top of the tool housing. One common type of blow out preventer includes a shear/blind ram in combination with one or two wireline rams. The shear ram is used to cut the wire or cable in an emergency. As described in U.S. Pat. No. 4,938,2909, the wireline ram(s) are designed to grip and hold the wire and include facilities for grease injection. The main disadvantage with these is their large size and weight. The weight, mounted on top of up to a 30 meters column, exerts a large bending moment on the lubricator and necessitates a stronger (and therefore heavier) tool housing and connectors.
A stuffing box is also normally included in a lubricator assembly above the grease injector. The stuffing box is intended to grip and hold the wire or cable in the event of gas leaking past the grease injector. Examples of known stuffing boxes are shown in UK Patent No. GB 2,214,954 and U.S. Pat. No. 2,943,682. In Pat. No. U.S. Pat. No. 5,863,022, a stripper/packer having a split bonnet is shown. The packer also serves as a blowout preventer. The packer can be axially activated to achieve a radial sealing, and the function of the packer is similar to a stuffing box. The present invention can be used together with a packer of this type.
To reduce some of the the weight the lubricator described in WO 0125593 uses only a shear/blind ram in conjunction with a second high pressure stuffing box with grease injection, the stuffing box being a replacement for the wireline ram. However, a stuffing box in this position will have well pressure acting on the lower surface of its rubber cylinder, thereby adding to the forces keeping the rubber in compression. There are also higher frictional forces. This makes it difficult to control the stuffing box properly. One consequence has been that it has proved difficult to reopen the stuffing box, forcing the operator to cut the wire and retrieve the whole lubricator to the surface. This can be a costly operation.
In U.S. Pat. No. 6,394,460, a one-piece ram element block for wireline blowout preventers is shown. The ram element block is a part of a BOP housing having a generally vertically oriented bore for a wireline. The BOP housing defines a pair of opposed ram element bores wherein linearly movable ram elements are located. Here, high pressure grease is injected into the flow passage between the upper and lower ram elements, thereby effecting a proper wireline seal when the rams are actuated to their closed positions. The ram elements are not located in separate bores and can not be independently controlled.
An object of the present invention is therefore to produce a pressure containment device in place of the stuffing box which can be positively and exactly balanced and will give a better control over the gripping forces than existing stuffing boxes. As the shear/blind ram function is also built into the device it will eliminate the need for the upper blowout preventer. It is small and compact and will therefore reduce the overall bending moments on the lubricator. This in turn makes it possible to reduce the strength and size of the tool housing and connectors.
The present invention utilises positively closing and opening rams to grip and hold the cable or wire. It also includes a shear/blind ram so that it will cut the wire or cable in a emergency. Because the unit is located in the pressure control head, e.g. above the tool housing, the internal size can be related to the wire diameter and not, as in the present, the tool diameter.